Geochemical Characteristics of A-Type Granite near the Hongyan Cu-Polymetallic Deposit in the Eastern Hegenshan-Heihe Suture Zone, NE China: Implications for Petrogenesis, Mineralization and Tectonic Setting

Abstract

In the eastern Hegenshan-Heihe suture zone (HHSZ) of NE China, Cu-Au hydrothermal mineralization at the newly discovered Hongyan deposit is associated with the Shanshenfu alkali-feldspar granite (SAFG). Zircon U-Pb dating showed that the inner phase and outer phase of the SAFG were formed at 298.8 ± 1.0 Ma and 298.5 ± 1.0 Ma, respectively. Whole rock geochemistry suggests that the SAFG can be classified as an A-type granite. Halfnium isotopes and trace elements in zircon suggest that the SAFG has high Ti-in-zircon crystallization temperature (721–990 °C), high magmatic oxygen fugacity and largely positive ε_Hf(t) (from +6.0 to +9.9). We proposed that the SAFG was derived from crustal assimilation and fractional crystallization of juvenile crust metasomatized by subducting oceanic crust. The high oxygen fugacity of the SAFG suggests the chalcophile elements (e.g., Cu, Au) remained in the magma as opposed to the magma source. An arc-related juvenile source favors enrichment of Cu and Au in the resulting magma. Combined, these magmatic characteristics suggest Cu ± Au exploration potential for magmatic-hydrothermal mineralization related to the SAFG, and similar bodies along the HHSZ. The results obtained combined with regional geological background suggest that the Permian A-type granites and related mineralization along the HHSZ were formed in a post-collisional slab break-off process.

1. Introduction

Since the introduction of the A-type granite classification by Loiselle and Wones [1], rocks of this type have been studied extensively due to their important geodynamic significance, complicated petrogenesis and economic potential [2,3,4,5,6,7,8,9,10,11,12,13]. It has been recognized that A-type granites can be formed in a variety of extensional regimes, such as continental back-arc extension, post-collisional extension or within-plate settings [2,14]. In recent years, there have been many studies on the origin and oxidation state of A-type granites and their relationships to mineralization [7,8,9,10,12,13]. In general, previous workers have related oxygen fugacity and a degree of magma contamination to different types of mineralization [7,8,9,10]. High magmatic oxygen fugacity and juvenile magma sources such as mantle-derived crust or partial melting of an oceanic slab have been related to Cu-Mo-Au-Pb-Zn mineralization [7,8,10]. Low magmatic oxygen fugacity, and wall-rock interaction or assimilation of continental crust are associated with W-Sn mineralization [7,8,9]. The oxidation state and petrogenesis of A-type granites has important geological significance, not only for regional tectonic evolution, but also for understanding the metallogenic process related to A-type granites. Decoding this petrogenetic intricacy, inherent geodynamic background and the related metallogenic process requires the use of an integrated whole rock geochemical and zircon chemical approach. Zircon chemistry relates to crystallization temperature and oxygen fugacity (fo₂) of magma [15,16,17,18], the composition of parental melts [19,20], source-rock type and crystallization environment [21,22,23]. The refractory nature and low elemental diffusion [24] mean that zircon chemistry remains unchanged through most geological processes, such as hydrothermal alteration, providing a powerful tool for documenting magmatic conditions (e.g., magma source and fo₂) and the resulting behavior of elements such as Cu [25,26,27,28].

In northeastern China (NE China), numerous Permian A-type granitoids (260–300 Ma) have been identified along the Hegenshan-Heihe suture zone (HHSZ) in the past two decades (Figure 1a,b) [3,4,29,30,31,32]. These A-type granites are on the eastern side of the larger Central Asian Orogenic Belt (CAOB, Figure 1a). Previous studies have provided mineralogical and geochemical constraints on the types (A2 type), sources (crust-mantle mixing or crustal sources) and tectonic settings (post-orogenic extension or post-collisional extension) of the Permian A-type granites along the HHSZ [3,4,29,30,31,32]. Newly discovered hydrothermal deposits are associated with these Permian A-type granites, including Aoyoute Cu-polymetallic deposit (287 Ma), Bayandulan Cu-polymetallic deposit (284 Ma) and Ataiwula Cu-polymetallic deposit (276 Ma) [32]. Despite a clear relationship between Permian A-type granitoids and mineralization, there is debate about the petrogenesis and tectonic setting of these rocks, and influence of the magma source and magmatic oxygen fugacity on the related metallogenic process needs further research. The newly discovered Hongyan Cu-polymetallic deposit is located in the northeastern part of the HHSZ (Figure 1c), and contains Cu-polymetallic hydrothermal mineralization clearly associated with a Permian A-type granitoid. The deposit and surrounding rocks provide a good opportunity to study the petrogenesis and tectonic setting of Permian A-type granitoids and their possible relationship with hydrothermal mineralization.

In this paper, we present zircon U-Pb ages, trace elements data and Hf isotope data with whole rock major and trace element data from Permian A-type granite in the Hongyan Cu-polymetallic deposit. We then use these data to discuss the oxidation state, petrogenesis and tectonic setting of these rocks. The results provide new insights into the petrogenesis and tectonic setting of Permian A-type granites along the HHSZ, and highlight the exploration potential of these rocks.

2. Regional Geology

The Xing-Meng Orogen Belt (XMOB) is located in the eastern part of the CAOB, and was formed in the Permian through the collision between the Siberia block and North China block (Figure 1a) [34,36,37]. The XMOB is further subdivided by several NE-striking faults into different microcontinental massifs or terranes, including the Ergun massif, the Xing’an terrane, the Songliao terrane, the Jiamusi terrane and the Liaoyuan terrane (Figure 1b). These terranes have undergone EW-trending tectonic evolution of the Paleo-Asian Ocean during the Paleozoic and NNE-trending tectonic evolution of the western Pacific Ocean during the Mesozoic and Cenozoic.

The HHSZ is a suture zone resulting from the collision of the Xing’an and Songliao terranes (Figure 1b). The outcropping strata in the northern Xing’an terrane and its adjacent area can be approximately divided into four units (Figure 1c) [38,39]: (1) Sporadic distribution of Precambrian crystalline basement rocks composed of granulites, gneisses and schists; (2) extensive exposure of early Paleozoic metamorphosed volcanic and sedimentary rocks consisting of schist, sandy slate, marble and andesite, which are formed in continental margin and arc accretion settings; (3) the late Paleozoic units are similar to the early Paleozoic units but with lower metamorphic grades, such as the extensive distribution of the Baoligaomiao volcanic succession that represents Carboniferous subduction-related, mature, continental arc volcanism [40]; and (4) Mesozoic continental intermediate-felsic volcanic and sedimentary rocks, and Cenozoic sedimentary basin strata. Intense magmatic activity occurred along the HHSZ, which can be divided into the Caledonian, Hercynian, Indosinian and Yanshanian periods [41]. The Caledonian granitoids, consisting of granodiorite and quartz diorite, are sporadically distributed in the Luomahu and Duobaoshan areas. The Hercynian granitoids including granodiorite, monzogranite and alkali-feldspar granite are widely distributed on the northern side of the HHSZ. The Indosinian granitoids, consisting of monzogranite, syenogranite and two-mica granite, are widely distributed on both sides of the HHSZ. The Yanshanian granitoids, being composed of quartz diorite, granodiorite, tonalite and biotite granite, exhibit a sporadic distribution (Figure 1c) [4,42].

Intense hydrothermal activity was associated with these periods of magmatism, and resulted in various types of hydrothermal deposits, primarily including skarn Cu-polymetallic deposits (e.g., Sankuanggou and Xiaoduobaoshan), porphyry Cu-(Mo-Au) deposits (e.g., Duobaoshan and Tongshan) and epithermal Au deposits (e.g., Shangmachang, Beidagou, Sandaowanzi, Tianwangtaishan and Zhenguang, Figure 1c) [34,43,44], which constitute the northeastern segment of the Xing’an Cu-Mo-Fe-Pb-Zn-Au belt [37,45]. These deposits are located in a nearly NE-trending band along the HHSZ and its secondary NE- and NW-trending faults (Figure 1c).

3. Deposit Geology

The Hongyan Cu-polymetallic deposit comprises three orebodies (from E125°01′28″ to E125°06′30″, from N49°33′30″ to N49°42′30″; Figure 2), located in the northeastern part of the HHSZ (Figure 1c). Paleozoic and Mesozoic volcanic rocks are widespread near the deposit (Figure 2). Paleozoic volcanic rocks mainly include the Upper Carboniferous–Lower Permian Baoligaomiao Formation, which is a suite of continental volcanic rocks composed of rhyolitic tuff, andesitic tuff and basalt [46]. Mesozoic volcanic rocks are subdivided into the Lower Jurassic Manitu Formation, the Upper Cretaceous Baiyingaolao Formation and Damoguaihe Formation. The Manitu Formation comprises dacitic ignimbrite and dacite. The Baiyingaolao Formation is mainly composed of rhyolite, rhyolitic volcanic breccias and rhyolitic ignimbrite. The Damoguaihe Formation is mainly composed of conglomerate, fine sandstone and tuff breccias [46]. Steeply dipping brittle faults are dominantly NE-trending (e.g., F1 and F2), NWW-trending (e.g., F3, F4, F5, F6, F7 and F9) and E-trending faults (e.g., F8). The Shanshenfu alkali-feldspar granite (SAFG) with an exposed area of ~75 km² dominates plutonic rocks. The SAFG intrudes Paleozoic volcanic rocks with an irregular contact and is covered by Mesozoic volcanic rocks and Quaternary sediments (Figure 2).

The SAFG comprises an inner medium-grained phase and outer fine-grained phase (Figure 2). The boundary between the phases is gradational. The inner phase contains subhedral perthite (50%–55%), anhedral quartz (30%–35%), plagioclase (5%–10%), hornblende (0%–5%) and muscovite (0%–5%, Figure 3a and Figure 4a). The outer phase comprises subhedral perthite (55%–60%), anhedral quartz (25%–30%), plagioclase (5%–10%), hornblende (0%–5%) and muscovite (0%–5%, Figure 3b and Figure 4b). Accessory minerals are magnetite (Figure 4d), zircon and apatite.

The SAFG is pervasively altered by potassic (Figure 3c) and quartz-sericite alteration (Figure 4c), and is accompanied by some disseminated Cu mineralization (Figure 4d). Cu-polymetallic mineralization generally occurs as quartz veins and veinlets in the SAFG and the Baoligaomiao Formation, which is structurally controlled by tensile cracks and NWW-striking faults, respectively (Figure 2). Three vein-shaped mineralized bodies are identified in the Hongyan Cu-polymetallic deposit, which from north to south are No. I, II and III, respectively [46] (Figure 2): The No. I ore body hosted in the outer phase of the SAFG develops NWW-trending ore veins with a length of 250 m, an average thickness of 3.1 m and Cu grades of 0.5%–1.5%; the No. II ore body hosted in the Baoligaomiao Formation develops NWW-trending and N-dipping ore veins with a length of 220 m, an average thickness of 2.3 m and Cu, Au and Ag grades of 0.4%–1.2%, 0.34–2.21 g/t and 0.76–21.6 g/t, respectively; the No. III ore body hosted in the Baoligaomiao Formation develops NWW-trending and N-dipping ore veins with a length of 215 m, an average thickness of 1.8 m and Cu, Au and Ag grades of 0.5%–1.35%, 0.27–2.62 g/t and 0.64–16.2 g/t, respectively.

4. Mineralization and Alteration in Ore Bodies

Based on our field and petrographic observations, the main ore minerals are chalcopyrite and pyrite, with minor bornite, covellite, galena, sphalerite, magnetite and native gold. The most common gangue minerals are quartz, K-feldspar, sericite, muscovite, chlorite, epidote, calcite and fluorite. Silicification is well developed and closely related to sulfide mineralization. Based on hydrothermal alteration mineral assemblages, we identify three stages of mineralization (Figure 5): Quartz ± K-feldspar ± pyrite (Stage I), quartz + chalcopyrite ± pyrite ± bornite ± sphalerite ± galena (Stage II) and quartz + carbonate ± fluorite (Stage III). Stage I is widely distributed in No. I and II ore bodies, which is defined by the occurrence of quartz and K-feldspar with minor pyrite and magnetite as veins (Figure 3c and Figure 4e). Stage II is the main ore-forming stage and mainly distributed in the three ore bodies, which is represented by the widespread occurrence of quartz, chalcopyrite and pyrite minerals as veins or veinlets with minor bornite, covellite, galena and sphalerite (Figure 3d–g and Figure 4f–h). Chalcopyrite replaces early bornite and is commonly intergrown with pyrite and galena (Figure 4g,h). Chalcopyrite and pyrite are partly replaced by covellite (Figure 4f). Gold mineralization is most commonly hosted within sulfide minerals in this stage, and visible gold can be found in the quartz veins (Figure 4i). Stage III is mainly distributed in No. II and III ore bodies, which is characterized by the appearance of calcite and quartz with minor fluorite as veins or veinlets (Figure 3h–i). Small amounts of pyrite occur in quartz-calcite veins, and gold mineralization is less in this stage. The presence of fluorite mineralization has been reported to be closely related to A-type granites [47,48]. Combined, the development of Cu-polymetallic mineralization in the SAFG and widespread K-feldspar alteration and fluorite mineralization in ore bodies suggest that these ore-forming hydrothermal fluids are mainly derived from the SAFG.

5. Analytical Methods

5.1. Zircon U-Pb Dating Analysis

The sample SF-1 from the inner phase and the sample SF-5 from the outer phase were collected for geochronology (Figure 2). Zircon grains were separated using conventional heavy liquid and magnetic techniques, and then handpicked under a binocular microscope. Handpicked zircon grains were mounted on adhesive tape, enclosed in epoxy resin and polished so that approximately half of each zircon is exposed. To study interior textures of zircons cathodoluminescence (CL) images were collected with a JEOL JXA-8100 electron microprobe (TESCAN MIRA 3 LMH FE-SEM, TESCAN, Brno, Czech Republic) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. Zircon U-Th-Pb measurements were done under a 32 μm diameter laser beam at the same laboratory using a Geo-Las 2005 System. An Agilent 7500a ICP-MS (Agilent, Santa Clara, CA, USA) instrument was used to acquire ion-signal intensities with a 50 mJ/pulse 193 nm ArF-excimer laser at 10 Hz and a homogenizing, imaging optical system (MicroLas, Göttingen, Germany). The data acquisition for each analysis took 100 s (40 s on background and 60 s on signal). Detailed instrumentation and analytical accuracy description were given by Liu et al. [49,50]. Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using a linear interpolation (with time) for every six analyses according to the variations of external standard zircon 91,500 (2 zircon 91,500 + 6 samples + 2 zircon 91,500) [50]. The ages are calculated by inhouse software ICPMSDataCal (version 6.9, China University of Geosciences) [49] and concordia diagrams were made by Isoplot/Ex ver3.0 [51]. Errors on individual analyses are quoted at the 1σ level, whereas errors on weighted mean ages are quoted at the 2σ (95% confidence) level. Trace element compositions of zircon are calibrated against GSE-1G combined with internal standardization ²⁹Si [50].

Zircon Ce and Eu anomalies, Ti-in-zircon temperatures and magmatic oxygen fugacity (fo₂) were calculated using the trace element compositions of zircons collected during the same analysis interval as U-Pb dating. The method for calculating these parameters has been described in Ferry and Watson [16] and Trail et al. [17,18]. CGDK software [52] was used for plotting data.

5.2. Zircon Lu-Hf Isotopes Analysis

In-situ zircon Hf isotopic analyses were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) equipped with a Geolas 2005 excimer ArF laser ablation system (Lambda Physik, Göttingon, Germany) at the state Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). All data were acquired on zircon in single spot ablation mode at a pulse rate of 20 Hz at 200 mJ with a spot size of 44 μm in this study. Each measurement consisted of 20 s of acquisition of the background signal followed by 50 s of ablation signal acquisition. Detailed operating conditions for the laser ablation system and the analytical method are the same as description by Hu et al. [53]. The ¹⁷⁶Hf/¹⁷⁷Hf ratios of the standard zircon (GJ1) were 0.282013 ± 0.000022 (2σ, n = 276), in agreement with recommended values within 2σ error [54,55]. Offline selection and integration of analyte signals, and mass bias calibrations were performed using ICPMSDataCal [50].

5.3. Whole-Rock Major and Trace Element Analyses

Three samples from the inner phase (from SF-1 to SF-3) and three samples from the outer phase (from SF-4 to SF-6) were collected for major and trace element determinations. All the samples are unaltered and unweathered. Geochemical analyses were carried out at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIG-CAS). Major element oxides were analyzed using a Rigaku RIX 2000 X-ray fluorescence spectrometer (XRF, Panalytical, Almelo, The Netherlands), and analytical uncertainties are mostly between 1% and 5% [56]. Trace elements were obtained by inductively coupled plasma-mass spectrometry (ICP-MS, Perkin Elmer Elan 9000, Perkin, Waltham, MA, USA) after acid digestion of samples in high-pressure Teflon vessels, and detailed procedures are described by Li et al. [56]. The USGS and Chinese National standards AGV-2, GSR-1, GSR-2, MRG-1, BCR-1, W-2 and G-2 were chosen for calibrating element concentrations of the analyzed samples. Analytical precision of REE and other incompatible element analyses is typically 1%–5%.

6. Results

6.1. Zircon U-Pb Ages

The zircon U-Pb ages for 23 zircons are shown in

Table 1. Zircon LA-ICP-MS U-Pb data of the Shanshenfu alkali-feldspar granite.

Spot	Trace Elements (ppm)			Isotopic Ratio						Apparent Age (Ma)
	U	Th	Th/U	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/206Pb	1σ	206Pb/238U	1σ	207Pb/235U	1σ	207Pb/206Pb	1σ
Sample SF-1
SF1-1	635	337	0.53	0.3362	0.0039	0.0472	0.0002	0.0517	0.0006	297	1.7	294	4.0	272	21.3
SF1-2	591	349	0.59	0.3425	0.0046	0.0475	0.0002	0.0523	0.0007	299	1.9	299	4.6	298	−2.8
SF1-3	817	683	0.84	0.3341	0.0035	0.0473	0.0002	0.0512	0.0005	298	1.8	293	3.8	250	24.1
SF1-4	656	345	0.53	0.3413	0.0046	0.0475	0.0001	0.0521	0.0007	299	1.6	298	4.6	300	31.5
SF1-5	1413	809	0.57	0.3434	0.0024	0.0473	0.0001	0.0526	0.0004	298	1.6	300	3.0	322	14.8
SF1-6	495	262	0.53	0.3403	0.0052	0.0475	0.0002	0.0519	0.0008	299	1.8	297	5.1	283	30.6
SF1-7	1130	738	0.65	0.3468	0.0029	0.0476	0.0002	0.0528	0.0004	300	2.0	302	3.4	320	16.7
SF1-8	988	482	0.49	0.3463	0.0031	0.0476	0.0002	0.0528	0.0005	300	1.8	302	3.5	320	20.4
SF1-9	822	430	0.52	0.3465	0.0033	0.0476	0.0002	0.0528	0.0005	300	1.9	302	3.7	320	20.4
SF1-10	1202	673	0.56	0.3418	0.0024	0.0475	0.0002	0.0522	0.0004	299	2.0	299	3.0	295	14.8
SF1-11	191	144	0.76	0.3417	0.0124	0.0473	0.0002	0.0523	0.0019	298	2.0	298	11.1	298	79.6
SF1-12	719	283	0.39	0.3428	0.0046	0.0475	0.0002	0.0523	0.0007	299	1.8	299	4.6	298	−2.8
Sample SF-5
SF5-1	613	326	0.53	0.3459	0.0037	0.0476	0.0002	0.0527	0.0005	300	1.9	302	3.9	317	22.2
SF5-2	215	148	0.69	0.3457	0.0103	0.0475	0.0002	0.0527	0.0015	299	1.8	302	9.2	317	69.4
SF5-3	698	481	0.69	0.3456	0.0044	0.0475	0.0002	0.0528	0.0006	299	2.0	301	4.4	320	25.9
SF5-4	680	214	0.31	0.3413	0.004	0.0473	0.0001	0.0524	0.0006	298	1.6	298	4.1	302	25.9
SF5-5	1062	490	0.46	0.3397	0.0027	0.0475	0.0002	0.0519	0.0004	299	1.7	297	3.2	280	18.5
SF5-6	239	144	0.6	0.3451	0.0087	0.0475	0.0002	0.0526	0.0013	299	1.8	301	7.9	309	55.6
SF5-7	1077	67	0.06	0.3433	0.0025	0.0472	0.0001	0.0527	0.0004	298	1.6	300	3.1	317	23.1
SF5-8	201	95	0.47	0.3472	0.0142	0.0476	0.0001	0.0531	0.0022	300	1.7	303	12.5	332	92.6
SF5-9	1407	651	0.46	0.3501	0.0024	0.0472	0.0001	0.0538	0.0003	297	1.6	305	3.1	365	13.0
SF5-10	583	243	0.42	0.3407	0.0058	0.0472	0.0001	0.0523	0.0009	297	1.7	298	5.5	298	41.7
SF5-11	1213	646	0.53	0.3422	0.0027	0.0473	0.0001	0.0525	0.0004	298	1.6	299	3.2	306	12.0

. Most of the zircon grains from two samples (SF-1 and SF-5) are euhedral and prismatic, and are relatively transparent and colorless. They have a length between 61 μm and 179 μm, with length to width ratios between approximately 1:1–3:1 (Figure 6a,b). CL images show that most zircons have magmatic oscillatory overgrowth rims, although a few zircons with high uranium contents are dark brown and turbid (Figure 6a,b). Most zircons have high Th/U ratios >0.5, which are typical of an igneous origin. Twelve spot analyses of zircons from sample SF-1 yield ²⁰⁶Pb/²³⁸U ages of 297 Ma to 300 Ma (Figure 6c) with a weighted mean age of 298.8 ± 1.0 Ma (MSWD = 0.27) for all 12 analyses. Eleven spot analyses from sample SF-5 yield ²⁰⁶Pb/²³⁸U ages between 297 Ma and 300 Ma (Figure 6d) with a weighted mean age of 298.5 ± 1.0 Ma (MSWD = 0.36) for all 11 analyses. These ages are coeval and are interpreted as the crystallization age for the SAFG.

6.2. Zircon Trace Element, Ti-in-Zircon Temperature and Oxygen Fugacity

Trace element compositions of zircons are listed in

Table 2. Zircon trace element abundance, Ce and Eu anomalies, Ti-in-zircon temperatures (T) and magmatic oxygen fugacity (fo₂) of the Shanshenfu alkali-feldspar granite.

Spot	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	ΣREE	Ti	δCe	δEu	TTi	lg(fo2)	FMQ Buffer	△FMQ
	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm	ppm			°C
Sample SF-1
SF1-1	1.57	268	1.39	21.3	53.9	0.6	333	119	1451	510	2244	417	3937	663	10,019	16.3	39.2	0.01	837	−10.1	−13.7	3.6
SF1-2	2.23	103	1.63	18.4	39.1	0.32	224	82.2	1050	395	1808	346	3297	566	7931	33.2	11.7	0.01	917	−11.5	−12.2	0.7
SF1-3	0.88	44.6	1.23	15.5	27.3	1.78	135	43.3	506	182	829	159	1586	284	3815	58.3	9.3	0.09	990	−9.9	−11.1	1.2
SF1-4	6.60	92.0	2.77	15.9	15.0	0.47	95.2	33.9	450	176	854	172	1798	321	4026	22.6	4.7	0.04	872	−16.7	−13.1	−3.6
SF1-5	0.11	161	0.74	19.8	58.6	0.26	348	126	1590	570	2490	457	4227	679	10,727	19.1	120.8	0.01	854	−5.2	−13.4	8.2
SF1-6	210	750	84.6	433	154	2.44	280	67.4	674	217	934	179	1774	312	5861	24.6	1.2	0.04	882	−21.3	−12.9	−8.4
SF1-7	0.24	116	0.5	9.2	44.4	1.84	251	86.8	1097	385	1700	316	2933	475	7416	19.2	73.2	0.05	855	−7.0	−13.4	6.4
SF1-8	0.04	137	0.75	7.9	27.8	0.59	186	70.9	947	350	1602	306	2990	504	7130	8.6	174.8	0.03	773	−7.4	−15.1	7.7
SF1-9	0.49	152	1.45	14.3	27.2	0.61	163	59.1	757	281	1317	261	2633	443	6110	9.3	39.0	0.03	781	−12.6	−14.9	2.3
SF1-10	0.07	191	0.34	7.6	21.2	0.33	162	67.4	904	338	1576	297	2838	452	6855	13.9	270.5	0.02	820	−3.6	−14.1	10.5
SF1-11	5.14	147	2.31	18.6	33.7	0.28	208	81.2	1073	406	1859	362	3546	585	8322	16.7	9.2	0.01	840	−15.4	−13.7	−1.7
SF1-12	0.02	72.7	0.17	4.3	17.9	0.53	110	43.5	589	229	1105	224	2242	367	5005	14.2	276.0	0.04	823	−3.4	−14.0	10.6
Sample SF-5
SF5-1	3.49	99.6	2.87	32.7	42.1	1.53	198	60.2	707	237	1038	204	1995	334	4952	33.6	6.8	0.05	919	−13.5	−12.2	−1.3
SF5-2	0.50	184	0.49	9.6	30.9	0.11	180	69.9	936	358	1607	321	3265	500	7462	32.5	80.7	0.00	915	−4.3	−12.3	8.0
SF5-3	12.2	212	6.04	35.3	36.1	0.24	199	75.3	987	369	1676	336	3362	528	7822	12.1	5.3	0.01	807	−18.9	−14.4	−4.5
SF5-4	5.38	183	2.59	22.3	36.9	0.49	232	92.7	1142	382	1685	338	3349	488	7954	9.2	10.6	0.02	780	−17.6	−14.9	−2.7
SF5-5	0.16	135	0.11	6.7	20.7	0.51	168	63.6	876	320	1488	290	2977	431	6777	20.4	215.0	0.03	861	−2.7	−13.3	10.6
SF5-6	0.31	161	1.34	17.9	38.1	0.27	216	76.6	962	347	1561	320	3247	502	7450	18.9	53.8	0.01	853	−8.3	−13.4	5.1
SF5-7	57.0	260	19.2	84.4	38.8	1.45	171	57.9	748	271	1258	255	2667	409	6241	9.8	1.7	0.05	786	−24.2	−14.8	−9.4
SF5-8	1.14	156	0.83	13.9	30.1	0.69	188	74.1	970	357	1595	324	3276	473	7459	18.4	34.8	0.03	850	−10.0	−13.5	3.5
SF5-9	0.36	87.4	0.42	6.8	15.1	0.27	82.6	27.8	370	134	627	135	1424	225	3135	173	48.3	0.02	1158	-	-	-
SF5-10	0.20	20.0	0.35	3.4	6.19	1.98	42.1	13.2	160	60.9	285	59.2	651	107	1410	33.5	16.4	0.38	918	−10.2	−12.2	2.0
SF5-11	4.03	109	1.65	14.6	30.1	0.78	204	78.5	1068	403	1890	376	3683	585	8444	4.8	9.2	0.03	721	−21.0	−16.3	−4.7

Note: T_Ti (°C) is the Ti temperature of zircon, δCe = Ce_N/(La_N × Pr_N)^0.5, δEu = Eu_N/(Sm_N × Gd_N)^0.5; FMQ buffer is the buffer of the fayalite-magnetite-quartz, which is calculated using the formula FMQ buffer = −24,441.9/(T + 273) + 8.290 (±0.167) [59]; △FMQ = lgfo₂ − FMQ buffer.

, and the corresponding chondrite-normalized REE patterns are plotted in Figure 7. The zircons of sample SF-1 and SF-5 have ΣREE contents ranging from 3815 ppm to 10,727 ppm and 1411 ppm to 8447 ppm, respectively, Ce anomalies ranging from 1.2 to 276 (average of 86) and from 1.7 to 215 (average of 44), respectively, Eu anomalies ranging from 0.01 to 0.09 and from 0 to 0.38, respectively and Ti contents ranging from 8.6 ppm to 58.3 ppm and from 4.8 ppm to 173 ppm, respectively. Typical igneous zircon has Ti values ≤75 ppm [21]. One grain in our study has high Ti value (SF5-9 = 173 ppm). A value this high is unusual for igneous zircon and may be the result of inclusions (e.g., ilmenite) in the analysis; this analysis is excluded from the following discussion. The temperature of the melt during zircon crystallization was calculated by the Ti-in-zircon thermometer [16] as shown in the following Equation (1):

$$T_{zircon}\left({}^{°}\mathrm{C}\right)=4800/\left(5.711-\lg (T{i}_{zircon}\right)-\lg \left(S{i}_a\right)+\lg \left(T{i}_a\right))-273$$

(1)

As quartz is one of the major mineral phases in the SAFG, the activity of silica (Si_a) is set to 1. Due to the absence of rutile in the SAFG, the activity of titanium (Ti_a) is conservatively estimated to be 0.6 [15,57]. The calculated Ti-in-zircon temperatures for the inner phase (sample SF-1) and the outer phase (sample SF-5) are in the range of 773 °C to 990 °C (average of 854 °C) and 721–919 °C (average of 841 °C, Table 2), respectively. A new calibration has been presented by Trail et al. [17,18] to determine the oxygen fugacity of magmatic melt based on the incorporation of cerium into zircon and Ti-in-zircon temperature, which can be expressed by the following empirical equation:

$$\ln {\left(\frac{\mathrm{Ce}}{{\mathrm{Ce}}^{*}}\right)}_D=\left(0.1156\pm 0.0050\right)\times \ln \left(f_{\mathrm{o}2}\right)+\frac{13860\pm 708}{T\left(\mathrm{K}\right)}-6.125\pm 0.484$$

(2)

where fo₂ represents oxygen fugacity, and T is absolute temperature calculated by revised Ti-in-zircon thermometry. The Ce anomaly can be estimated by the following equation:

$${\left(\frac{\mathrm{Ce}}{{\mathrm{Ce}}^{*}}\right)}_D\approx {\left(\frac{\mathrm{Ce}}{{\mathrm{Ce}}^{*}}\right)}_{\mathrm{CHUR}}=\frac{{\mathrm{Ce}}_{\mathrm{N}}}{\sqrt{{\mathrm{La}}_{\mathrm{N}}\cdot P{r}_N}}$$

(3)

where Ce_N, La_N and Pr_N are chondrite-normalized values for Ce, La and Pr in zircon, respectively.

The lg(fo₂) values for the zircons of the inner phase and the outer phase are in the range of −21.3 to −3.4 (average of −10.3) and from −24.2 to −2.7 (average of −13.1), respectively, with corresponding ΔFMQ values of −8.4 to +10.6 (average of +3.1) and from −9.4 to +10.6 (average of +0.7), respectively (Table 2).

6.3. Zircon Lu–Hf Isotopes

The analytical results on Hf isotopes for zircons are listed in

Table 3. LA-MC-ICP-MS zircon Lu-Hf isotope data for the Shanshenfu alkali-feldspar granite.

Spot	176Hf/177Hf a	1σ	176Lu/177Hf a	1σ	176Yb/177Hf a	1σ	Age (Ma)	εHf (t) b	1σ	TDM1 (Ma) c	TDM2 (Ma) d	fLu/Hf
Sample SF-1
SF1-1	0.282995	0.000015	0.002996	0.000008	0.086319	0.000235	297	7.9	0.74	384	507	−0.91
SF1-4	0.282982	0.000014	0.002209	0.000051	0.061855	0.001585	299	7.4	0.70	395	533	−0.93
SF1-7	0.283024	0.000015	0.004243	0.000064	0.115732	0.00164	300	8.9	0.75	353	450	−0.87
SF1-8	0.283052	0.000016	0.003983	0.000026	0.117902	0.00081	300	9.9	0.76	307	394	−0.88
SF1-9	0.283011	0.000014	0.003086	0.000051	0.089502	0.001556	300	8.4	0.72	362	476	−0.91
SF1-11	0.282955	0.000013	0.00128	0.000005	0.032593	0.000132	298	6.5	0.70	424	587	−0.96
SF1-12	0.282967	0.000014	0.002444	0.000009	0.068065	0.000324	299	6.9	0.73	420	563	−0.93
Sample SF-5
SF5-1	0.283011	0.000017	0.004065	0.000056	0.11864	0.001618	300	8.4	0.79	372	476	−0.88
SF5-2	0.283020	0.000014	0.002091	0.000018	0.055445	0.0004	299	8.8	0.72	338	458	−0.94
SF5-5	0.283029	0.000017	0.003066	0.000016	0.090641	0.000562	299	9.1	0.79	335	441	−0.91
SF5-6	0.282941	0.000014	0.000794	0.000014	0.020234	0.000404	299	6.0	0.70	438	614	−0.98
SF5-7	0.283002	0.000014	0.003232	0.000016	0.093923	0.000628	298	8.1	0.72	377	494	−0.90
SF5-8	0.282948	0.000015	0.001153	0.000078	0.029802	0.002258	300	6.2	0.73	433	602	−0.97
SF5-9	0.283015	0.000014	0.003446	0.00001	0.099972	0.000247	297	8.6	0.72	359	468	−0.90
SF5-10	0.283025	0.000014	0.002295	0.000037	0.066671	0.001076	297	8.9	0.72	333	448	−0.93
SF5-11	0.283028	0.000015	0.003929	0.00002	0.113707	0.000769	298	9.0	0.75	344	443	−0.88

^a The measured values; ^b Initial ¹⁷⁶Hf/¹⁷⁷Hf ratios were calculated using the measured ¹⁷⁶Lu/¹⁷⁷Hf ratios and the ¹⁷⁶Lu decay constant of 1.867 × 10⁻¹¹ year⁻¹ [60], and ε_Hf values were calculated using the chondritic ratios of ¹⁷⁶Hf/¹⁷⁷Hf (=0.282772) and ¹⁷⁶Lu/¹⁷⁷Hf (=0.0332) [61]; ^c The single-stage model age (T_DM1) was calculated using the present-day ratios ¹⁷⁶Hf/¹⁷⁷Hf = 0.28325 and ¹⁷⁶Lu/¹⁷⁷Hf = 0.0384 [62]; ^d Two-stage model age (T_DM2) was calculated by projecting the initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of the zircon back to the depleted mantle model growth curve, assuming a ¹⁷⁶Lu/¹⁷⁷Hf value of 0.015 for the average continent crust [63].

. The ε_Hf(t) values are calculated using their U-Pb ages. Seven spots from the inner phase show a range of initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.282955 to 0.283052 and ε_Hf(t) values from +6.5 to +9.9, with the two-stage Hf model ages (T_DM2) from 394 Ma to 587 Ma. Nine spots from the outer phase show a range of initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.282941 to 0.283029 and ε_Hf(t) values from +6.0 to +9.1, with T_DM2 from 441 Ma to 614 Ma.

6.4. Whole-Rock Major and Trace Element Analyses

The major- and trace-element compositions of the SAFG are shown in

Table 4. Major oxides and trace elements of the Shanshenfu alkali-feldspar granite.

Sample	The Inner Phase			The Outer Phase
	SF-1	SF-2	SF-3	SF-4	SF-5	SF-6
SiO2	76.83	75.23	76.02	76.22	74.89	75.36
Al2O3	12.34	12.56	12.78	12.67	12.85	12.54
Fe2O3	1.51	1.48	1.37	1.64	2.13	1.95
CaO	0.08	0.11	0.09	0.19	0.21	0.24
MgO	0.14	0.12	0.11	0.16	0.09	0.12
Na2O	3.71	4.08	3.87	3.67	4.23	4.13
K2O	4.46	4.31	4.42	4.87	4.53	4.68
TiO2	0.15	0.27	0.19	0.17	0.25	0.31
MnO	0.06	0.08	0.04	0.11	0.05	0.06
P2O5	0.03	0.08	0.04	0.06	0.11	0.08
LOI	0.45	0.67	0.54	0.24	0.48	0.37
Total	99.98	99.56	99.71	100.32	99.67	99.48
La	52.3	22.6	45.5	58.8	27.2	40.1
Ce	161	54.2	101.3	174.5	72.3	89.6
Pr	14.75	6.58	13.2	15.1	7.22	12.6
Nd	53.6	20.8	44.2	51.3	29.1	45.5
Sm	12.8	5.13	9.35	11.26	7.32	9.81
Eu	0.34	0.15	0.13	0.42	0.17	0.21
Gd	10.72	5.82	7.62	10.11	9.61	6.97
Tb	1.91	1.28	1.18	1.83	2.36	1.11
Dy	12.3	10.44	7.61	12.28	13.12	6.5
Ho	2.54	2.35	1.51	2.57	2.68	1.27
Er	8.57	7.42	4.83	7.93	6.68	5.24
Tm	1.25	1.11	0.68	1.17	0.97	0.72
Yb	8.65	8.03	4.97	8.21	7.32	5.16
Lu	1.4	1.28	0.83	1.32	1.15	0.88
Ga	21.2	19.7	22.6	24.3	21.8	18.9
Rb	177	153.5	172	181.5	167	148
Ba	60.2	107	171	58.6	123	86
Th	16.3	17.51	14.2	17.1	18.2	13.6
U	3.79	3.65	3.48	2.87	3.67	3.13
Ta	2.23	2.65	2.31	2.95	2.46	2.33
Nb	43.7	49.1	44.3	48.2	46.2	44.7
Sr	8.3	14.8	22.4	24.1	15.3	18.7
Zr	358	435	647	248	487	276
Hf	11.9	13.1	18.4	10.8	13.9	11.2
Y	70.4	75.1	81.8	54.2	78.3	63.1
ΣREE	342	147	243	357	187	226
ΔEu	0.09	0.08	0.05	0.12	0.06	0.08

as weight percent (wt %) for major oxides and as parts per million (ppm) for all trace elements. According to the QAP ternary diagram [64] (Figure 8a), all samples classify as alkali-feldspar granite. They are characterized by high SiO₂ (74.89–76.83 wt %), Al₂O₃ (12.34–12.85 wt %) and alkali earths (Na₂O + K₂O = 8.17–8.81 wt %) with low MgO (0.09–0.16 wt %) and CaO (0.08–0.24 wt %). They exhibit a high-K calc-alkaline, peraluminous and high-silica content, with A/CNK (molar Al₂O₃/CaO + Na₂O + K₂O) ranging from 1.04 to 1.12 and A/NK (molar Al₂O₃/Na₂O + K₂O) from 1.06 to 1.14 (Figure 8b,c). The inner phase has slightly higher SiO₂ content with lower Fe₂O₃, TiO₂, CaO and P₂O₅ than the outer phase (Table 4). This may indicate that SAFG cooled from the margin into the centre and that this cooling process resulted in a slightly more evolved inner phase. Both phases exhibit enriched REEs (except Eu) with total REE ranging from 147 ppm to 357 ppm. They are characterized by slightly enriched light REE (LREE, (La/Yb)_N = 2.02–6.57) and flat heavy REE (HREE, (Gd/Yb)_N = 0.6–1.27) with strong negative Eu anomalies (δEu = 0.05–0.12, Figure 9b). All samples are enriched in large-ion lithophile elements (LILEs, e.g., Rb, Th, U and K), high-field-strength elements (HFSEs, e.g., Nb, Ta, Zr and Hf), and exhibit strong depletion of Ba, Sr, P and Ti (Figure 9a).

7. Discussion

7.1. Genetic Type and Magmatic Oxygen Fugacity

The SAFG has high SiO₂ (74.89–76.83 wt %), total alkalis (K₂O + Na₂O = 8.17–8.81 wt %) and low P₂O₅ (0.03–0.11 wt %) contents (Table 4). It is enriched in HFSE (e.g., Nb, Ta, Zr and Hf) and REEs (except Eu), and is depleted in Sr, Ba, P, Ti and Eu (Figure 8c). These geochemical features are common for A-type granites [4]. An A-type granite classification is also supported by the elevated Ga content of all samples such that they plot in the A-type field on the discrimination diagram of Whalen et al. [67] (Figure 10a) and the within-plate-granite field on the diagram Y vs. Nb of Pearce et al. [68] (Figure 10b). Chondrite normalized REE patterns of zircons show they are depleted in LREE and enriched in HREE, and exhibit strong positive Ce anomalies (δCe = 1.22–275.99) and deep negative Eu anomalies (δEu = 0.1–0.38, Figure 7), which are typical of unaltered magmatic zircons [19]. Positive Ce anomalies may be related to a relatively oxidized melt with favorable incorporation of Ce⁴⁺ while negative Eu anomalies may be resulted from plagioclase fractionation in magma composition [17,18]. Due to the close relationship between the Ce anomaly in zircon and the oxidation state of the melt from which the zircon crystallizes [17,18], zircon can indicate the fo₂ of magmas. By plotting the Ti-in-zircon temperatures and the logarithmic oxygen fugacities on T vs. lg(fo₂) diagram (Figure 10c) that can be divided by the curves of some specific mineral oxidation buffers such as magnetite-hematite (MH), fayalite-magnetite-quartz (FMQ) and iron-wustite (IW) into several oxygen fugacity fields [17], nearly two thirds of the data points plot above the FMQ buffer (Figure 10c), which indicates oxidized magmatic conditions. The presence of magnetite (Figure 4d) and absence of significant amounts of ilmenite also suggest the oxidized magmatic conditions for the SAFG [12].

7.2. Petrogenesis and Implications for Mineralization

Several petrogenetic models have been proposed for the origin of A-type granites, including (1) extreme fractional crystallization of mantle-derived tholeiitic or alkaline basaltic magmas [6,69]; (2) low-degree partial melting of lower-crustal granulites with depletion of incompatible elements [70]; (3) anatexis of underplated I-type tonalitic crustal source [12,71,72]; and (4) hybridization between anatectic crustal and mantle-derived magmas, such as crustal assimilation and fractional crystallization of mantle-derived magmas, or of mixing between mantle-derived and crustal magmas [72,73,74]. The zircons of the SAFG exhibit high ε_Hf(t) values (from +6.0 to +9.9) and young T_DM2 (394–614 Ma, Table 3), which is consistent with juvenile crustal sources. Thus, we can rule out the possibility of partial melting of lower-crustal granulites, which would have a more isotopically evolved signature. The Ti-in-zircon temperatures of the SAFG are between 721–990 °C, which suggests that anatexis of underplated I-type tonalitic crust is unlikely to be the source because that process cannot provide the required high temperature needed for the formation of A-type granites [6,75]. In addition, this magma source is not compatible with the observed elemental variations (e.g., Nb, Ta, Ti, P, Sr and Eu) in the SAFG [66]. Due to the absence of spatially or temporally associated alkaline mafic rocks in the study area [76], alkaline basaltic juvenile crust is not a suitable source for the SAFG. Thus, Two petrogenetic models that may explain the characteristics reported here for the SAFG are discussed further: (1) Extreme fractional crystallization from mantle-derived tholeiitic magma, and (2) hybridization between anatectic crustal and mantle-derived magmas.

Numerous studies suggest that the high magmatic oxygen fugacity is closely related to the nature of magma source regions [30,77,78]. The SAFG not only has similar U-Pb age and ε_Hf(t) values to late Carboniferous arc intrusions from northern inner Mongolia (Figure 11a,b), but also has the high oxygen fugacity similar to arc magmas that can range from FMQ to FMQ + 6 [77]. The chemistry of zircon is related to the magma from which the zircon crystallized, two recent discrimination diagrams proposed by Grimes et al. [22] suggest zircon with relatively higher U or lower Yb or Y are sourced from continental crust, whereas zircon with lower U or higher Yb or Y are sourced from ocean crust. In Figure 11c,d, most of the studied zircons plot in the field of ocean crust zircon or in the intersection between continental zircon and ocean crust zircon. The evidence above suggest that arc-related juvenile crust is a possible source for the SAFG. This magma source of Permian A-type granites in the CAOB has been proposed by previous studies [76,79]. The depletion of Nb, Ta, Ti, P, Sr and Eu in the SAFG indicates contamination and metasomatism by subducting oceanic crust [80]. This is also consistent with peraluminous nature and muscovite reported in mineralogy. Therefore, we propose that the SAFG was derived from the crustal assimilation and fractional crystallization of juvenile crust metasomatized by subducting oceanic crust. This magma source is similar to charnockitized juvenile crust proposed by previous studies [79,81,82,83].

Based on related studies conducted on Cu-Au-Mo hydrothermal deposits, the link between oxidized felsic magmas and mineralization is well recognized [13,27,73,78,84]. Sun et al. [27,78] documented that sulfur in melt exists mainly as SO and SO₂ at high fo₂ and as S²⁻ at low fo₂. Numerous experiments suggested that the partition coefficients of chalcophile elements (e.g., Cu, Au and Mo) between silicate melt and pyrrhotite are positively correlated with fo₂ [84,85,86]. Therefore, high fo₂ of the SAFG favors transmission of the chalcophile elements (e.g., Cu and Au) of the source region into the melt together with oxidized sulfur, rather than into the sulfides (reduced sulfur) [17,25,26,38,78,87]. With the decrease of temperature and pressure, hydrous magma can release a water-rich volatile phase to form a magmatic-hydrothermal ore-forming system [88]. Mao et al. [89] revealed the magmatic-hydrothermal mineralization process of the SAFG in detail through the study of melt-fluid inclusions. In this process metals such as Cu can be effectively enriched in the fluid phases after magmatic fractional crystallization due to high fo₂ and high exsolution temperature of the SAFG [89], consequently facilitating subsequent Cu-polymetallic hydrothermal mineralization.

7.3. Extensional Setting for Cu-Polymetallic Mineralization

Recent studies suggest that Cu-polymetallic mineralization associated with extensional setting in the XMOB was mainly formed in the early Cretaceous [10,91,92,93]. The extensional setting for Cu-polymetallic mineralization in the Permian is rarely reported. Therefore, the clear tectonic setting of the Permian A-type granites associated with Cu-polymetallic mineralization along the HHSZ is of great significance for regional tectonic evolution and prospecting. The SAFG with an age of 298 Ma is not only contemporary with immense alkali granites from southern Mongolia [94], but also coincident with the equivalents in the central inner Mongolia [79] and Erguna-Xing’an terrane of NE China [4,34]. Eby [2] divided the A-type granites into A1 and A2 groups. The A1 group is emplaced in anorogenic settings such as plumes, hotspots or continental rift zones. The A2 group is related to a cycle of subduction-zone or continent-continent collision magmatism in crust and emplaced in a variety of tectonic settings. In our study, the SAFG plots in the A2 field on the Nb-Y-3Ga diagram of Eby [2] (Figure 12a), and plot in the post-collisional setting on the Y + Nb versus Rb plot of Pearce et al. [68] (Figure 12b). Such an environment is also supported by the previous studies for other Permian A-type granitoids along the HHSZ [4,78,95]. The SAFG together with the other Permian A2-type granites [3,4,29,30,31] are commonly found with Carboniferous calc-alkaline I-type plutons along the HHSZ [36]. The increase in granitoid alkalinity with time indicates the tectonic transition from an earlier arc setting to a later extensional setting [96]. Similar tendency also occurs in southern Mongolia [94,95] and central inner Mongolia [79].

Previous studies suggest that the ocean along the Hegenshan-Heihe was closed during the late Carboniferous, which could be proved by the following evidences: (1) The Hegenshan ophiolite was emplaced before the early Permian, most likely between 300 Ma and 335 Ma [97,98]; (2) Permian and Carboniferous submarine turbidite strata with predominant magmatic arc source exists along the HHSZ [99]. However, the last Paleo-Asian ocean was not closed along the HHSZ during the late Carboniferous but closed along the Solonker-Xra Moron-Changchun suture during the late Permian–early Triassic [100]. This is well evidenced by late Permian to early Triassic post-orogenic magmatic rocks form belts along the Solonker-Xra Moron-Changchun suture [73,75]. Taking into account the fact that the Permian A2-type granites (260–300 Ma) are strikingly sparse and small in volume, and show a NE-trending migration together with the widespread occurrence of Permian volcanic rocks of island arc affinity and late Permian terrestrial sediments along the HHSZ [4], the post-collisional slab break-off process along the HHSZ during the Permian was generally proposed by previous studies [74,101]. Such tectonic regime is also evidenced by contemporaneous thermal metamorphism recorded by the Xilinhot metamorphic complexes in northern inner Mongolia [36], which once represented a segment of fore-arc basin [36]. In addition, the corresponding change can be observed in sedimentary style and clastic provenance of the early Permian strata from central inner Mongolia [102] and southern Mongolia [103]. The hallmark features of post-collisional slab break-off event is well recorded in numerous Phanerozoic cases [72,101,104], including transition in magmatic affinity from I-type to A-type, a narrow linear zone of magmatism, thermal metamorphismand change in uplift rate and concomitant sedimentary style [105]. All these features fit relatively well with the corresponding expressions along the HHSZ during Permian [4,36,102,103]. This unusual tectonic regime not only terminates a prolonged northward subduction of the Paleo-Asian ocean, but also heralds the amalgamation between the Songliao terrane and the Liaoyuan terrane along the Ondor Temple-Xar Moron suture zone.

The age of ore-related A-type granite in the Hongyan Cu-polymetallic deposit is consistent with the mineralized A-type granites in the southwestern part of the HHSZ [32], which suggests that these A-type granites associated with mineralization are formed in the same extension setting. During the Permian, the HHSZ was characterized by post-collisional extensional tectonism with the slab break-off, which not only caused upwelling of asthenospheric mantle and resulted in partial melting of the overlying lithospheric mantle and the juvenile arc crust [79], but also provided the thermal flux to form some narrow extensional magmatic channels, subsequently followed by A-type magmatism and related mineralization deposition along the HHSZ. Thus, the Permian A-type granites with arc-related juvenile crustal source and high fo₂ along the HHSZ have great potential for prospecting Cu-polymetallic hydrothermal deposits, and deserve more attention in future exploration.

8. Conclusions

Based upon geochronological and geochemical data from the SAFG, we suggest the following:

(1) LA-ICP-MS U-Pb zircon dating shows the inner phase and outer phase of the SAFG were formed at 298.8 ± 1.0 Ma and 298.5 ± 1.0 Ma, respectively. This makes the SAFG an early Permian pluton.

(2) The SAFG is a typical A-type granite with high magmatic oxygen fugacity. The source for the SAFG magma includes both crustal assimilation and fractional crystallization of juvenile crust metasomatized by subducting oceanic crust.

(3) The oxidized magmatic conditions favor transport and enrichment of chalcophile elements (e.g., Cu and Au) while the arc-related juvenile source favors Cu and Au enrichment. Combined, these magmatic characteristics suggest related magmatic-hydrothermal mineralization may be restricted to Cu ± Au.

(4) Combined with regional geological background, the Permian A-type granites and related mineralization along the HHSZ were formed in post-collisional extensional tectonism with the slab break-off.